Method for encoding digital video data

ABSTRACT

The present invention relates to a method for encoding digital video data corresponding to a sequence of digital source images using a cache memory, each of the digital source images having an equal source image width corresponding to a first number of blocks, the cache memory having a cache width corresponding to a second number of blocks, wherein the second number of blocks is smaller than the first number of blocks.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/013,730, filed Feb. 2, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/271,076, filed May 6, 2014, which issued on Apr.5, 2016 as U.S. Pat. No. 9,307,253, which are incorporated by referenceas if fully set forth.

FIELD OF INVENTION

The present invention relates to a method for encoding digital videodata corresponding to a sequence of digital source images.

BACKGROUND

Reference cache memories are used to speed up the encoding process whenencoding digital video data corresponding to a sequence of digitalsource images. The reference cache memory comprises at least a part of areference image. If requested data is contained in the cache (cachehit), this request can be served by simply reading the cache, which iscomparatively faster. Otherwise (cache miss), the data has to berecomputed or fetched from its original storage location, which iscomparatively slower. Hence, the greater the number of requests that canbe served from the cache, the faster the overall system performancebecomes. To be cost efficient and to enable an efficient use of data,cache memories are relatively small in terms of memory size.

When using a reference cache memory data is transferred between theoriginal storage location and the cache memory in blocks of data offixed size, called cache lines, this is usually the smallest amount ofdata that can be read from the original storage location (typicalexternal DRAM memory). When encoding images the blocks of datacorresponds to blocks of pixels also called macro blocks. The datastored in the reference cache memory is hence stored in a structuredorder making it simple to fetch the image data in the cache memory.

Moreover, the reference cache memory, dedicated to be used when encodingdigital video data, is having a cache width corresponding to a maximumsource image width being storable in the cache memory.

When encoding digital video data the reference cache memory is arrangedto store a preselected number of blocks or macro blocks lines of thereference image. This information is used by a motion estimation unit tofind motion in between two frames of the digital video data. The numberof blocks or macro blocks lines may vary from e.g. 3 block/macro blocklines up to the block/macro block line count that is need for thatparticular application; a common value is 5 or 7. Moreover, thereference cache memory has to be large enough to store full lines of theblocks/macro blocks of the source image in order to functionefficiently. For each new block/macro block line in the source imagethat will be compressed, the reference cache memory has to be read atleast once to perform the motion search. When a source image block/macroblock line has been finished the oldest block/macro block line in thereference cache memory may be free to use for a new block/macro blockline further down in the reference image.

In order to optimally use the benefits of having a reference cachememory when encoding digital video data corresponding to a sequence ofdigital source images the source image width, measured in number ofblocks of data, must be smaller or equal to the cache width alsomeasured in number of blocks of data.

Thus, encoding source images having a comparatively large source imagewidth, e.g. 4K UHDTV (2160p) (3840×2160 pixels, corresponding to 240×135macro blocks) or 8K UHDTV (4320p) (7680×4320 pixels, corresponding to480×270 macro blocks) requires a cache memory having a comparativelylarge cache width in order to use the cache memory optimally for fastencoding of the digital video data. However, video devices of today donot comprise cache memories of this size.

The cache width of a video encoding device typically follows the sourceimage width of the video images that the video encoding device isencoding to ensure fast encoding of the digital video data. There ishowever a trend towards larger pixel counts of the source images bothdue to the fact that image sensors capturing source images have anincreasing number of sensor pixels as well as the fact that sourceimages may be composed of image data captured by several individualimage sensors. In video encoding devices encoding large source images itmay hence not be possible to have a cache memory matching the size ofthe source image for a number of reasons; large cache memory haverelatively high power requirements and are complicated and henceexpensive to fabricate, if they are available at all.

Hence, there is a need for optimal usage of existing reference cachememories for speeding up the encoding of comparatively large sizeddigital video data.

SUMMARY

In view of the above, an objective of the present invention is to find asolution to the above mentioned problem.

In particular, according to a first aspect of the invention a method forencoding digital video data corresponding to a sequence of digitalsource images using a digital video data encoder comprising a cachememory is provided. Each of the digital source images have an equalsource image width corresponding to a first number of blocks and thecache memory have a cache width corresponding to a second number ofblocks, wherein the second number of blocks is smaller than the firstnumber of blocks. Hence, the cache width being smaller than the sourceimage width. The method comprises encoding a frame of the digital videodata, the frame comprising data corresponding to a first part of adigital source image in the sequence of digital source images, the firstpart of the digital source image having an image width corresponding toa third number of blocks being smaller than or equal to the secondnumber of blocks such that data pertaining to the first part of thedigital source image is storable in the cache memory, and the framebeing padded with skip blocks such that an image width of the framecorresponds to the source image width, and encoding a subsequent frameof the digital video data, the subsequent frame comprising datacorresponding to a second part of the digital source image, the secondpart of the digital source image having an image width corresponding toa fourth number of blocks being smaller than or equal to the secondnumber of blocks such that data pertaining to the second part of thedigital source image is storable in the cache memory, and the subsequentframe being padded with skip blocks such that the image width of thesubsequent frame corresponds to the source image width.

Hence, according to the present invention an encoding method whichenables video devices to encode comparably large video images usingreference cache memories with a limited cache width is provided. Theencoding method according to the present invention enables usage ofcache memories having a cache width being at least half the size of thesource image width, by dividing the digital source images into more thantwo parts even smaller cache memories may be used. This enables thecache memory to continue to be comparatively small in terms of memorysize saving space on the chip onto which the cache memory is attached.Moreover, a comparatively small cache memory is cheaper to manufacturethan comparatively large cache memory. Moreover, the encoding methodaccording to the present invention may be executed using existingstandard digital video data encoders, e.g. being able to encodeaccording to any of the ISO/MPEG standards or the ITU-H.26X standards.Furthermore, by using the encoding method according to the presentinvention encoded digital video data is produced, wherein each frame ofthe encoded digital video data have an image width corresponding to theimage width of the source image width but a frame rate being a multipleof the frame rate of the sequence of digital source images. Such digitalvideo data may be decoded and played back using a standard decoder andvideo device by setting the frame rate of the video device to themultiple frame rate of the sequence of digital source images and settingthe image width to the source image width. This means that the generateddigital video stream will be fully compatible with existing decodingstandards and yet allowing the cache to be comparably small in terms ofmemory size.

The frame and subsequent frame of the digital video data may be encodedas INTER-frames.

The method may further comprise encoding a preceding frame of thedigital video data, the preceding frame corresponding to an additionaldigital source image in the sequence of digital source images, whereinthe preceding frame is encoded as an INTRA-frame. When encoding an INTRAframe, no search is needed, thus the cache is not used. INTRA frames areencoded according to the standard.

The method may further comprise providing an additional frame of thedigital video data, the additional frame being an INTER-frame comprisingonly skip blocks. The additional frame of the digital video data isintroduced so that a constant frame rate may be obtained. Hence afterthe INTRA frame there will be some totally empty frames (skip only) thenthe encoding of the “real” INTER frames will start.

The additional frame may be subsequent the preceding frame.

The frame and the subsequent frame may be subsequent the additionalframe.

The first and second parts of the digital source image may not overlap.This is optimal since the same data pertaining to the digital sourceimage only need to be encoded once.

The image width of the first and second parts of the digital sourceimage may be equal.

The digital source images of the sequence of digital source images maycomprise data acquired by a plurality of image sensors and wherein thefirst part of the digital source image corresponds to image dataacquired by a first image sensor and the second part of the digitalsource image corresponds to image data acquired by another image sensor.The image data from different sensors may be cropped, stretched,shrinked, warped, processed etc automatically or by calibration to alignand even out any overlap or imperfectness due to differences in thephysical sensor placements, optical effects etc. before encoding.

The source image may be divided into multiple parts where each part(horizontally) has a number of blocks that is small enough to fit in thereference cache. Hence, the source image width may be a multiple of thecache width.

The method may further comprise sending side information pertaining tothe encoded digital video data to a decoder arranged to decode theencoded digital video data, wherein the side information comprises atleast one of a frame rate of the encoded video data, a frame rate of thesequence of digital source images, the cache width and the source imagewidth to a decoder arranged to decode the encoded digital video data.Using this information the decoder may be optimized for displaying theencoded video data in an optimal manner. The sending of the sideinformation to the decoder may be made over a side channel, or byspecial messages in the bit stream or by manual configuration or preconfiguration. By sending side information according to the above aratio between the frame rate of the encoded video data and the framerate of the sequence of digital source images may be found. In thedecoder the ratio between the frame rates may be used to set whichframes of the decoded video data that shall be displayed. If for example4 times the framer rate of the sequence of digital source images wasused to encode the digital video sequence the decoder may choose not todisplay new information until all four parts of the original sequencehas been decoded. By using this alternative method of displaying thedecoded encoded video data the frame rate of the displayed video datacorresponds with the frame rate of the sequence of digital sourceimages.

According to a second aspect of the invention a computer-readablerecording medium having recorded thereon a program for implementing theabove method when executed on a device having processing capabilities isprovided.

It is noted that the invention relates to all possible combinations offeatures recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to appended drawings showing embodimentsof the invention. The figures should not be considered limiting theinvention to the specific embodiment; instead they are used forexplaining and understanding the invention.

FIG. 1 is a schematic view of a sequence of four frames of encodeddigital video data according to the present invention.

FIG. 2 is a schematic flow chart, showing a method for encoding digitalvideo data according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of explanation, specificnomenclature is set forth to provide a thorough understanding of thepresent invention. However, it will be apparent to one skilled in theart that these specific details are not required in order to practicethe present invention. For example, the present invention has beendescribed with reference to the video coding standards of the ISO/MPEGfamily (MPEG-1, MPEG-2, MPEG-4) and to the video recommendations of theITU-H.26X family (H.261, H.263 and extensions, H.264 and HEVC, theupcoming h.265 standard). However, the same techniques can easily beapplied to other types of video coding standards, e.g. Microsoft codecsbelonging to the WMV-family, On2 codecs (e.g. VP6, VP6-E, VP6-S, VP7 orVP8) or WebM.

In modern digital video coding systems, two main modes are used tocompress a video frame of a video stream of video frames: the INTRA-modeand the INTER-mode. In the INTRA-mode, the luminance and chrominancechannels are encoded by exploiting the spatial redundancy of the pixelsin a given channel of a single frame via prediction, transform andentropy coding. The INTER-mode, exploiting the temporal redundancybetween separate frames, relies on a motion-compensation predictiontechnique that predicts parts of a frame from one (or more) previouslydecoded frame(s) by encoding the motion in pixels from one frame to theother for selected blocks. In INTER mode the luminance and chrominancechannels does not have any motion predictions.

Usually, a frame to be encoded is partitioned into minimum coding units(block, macro blocks etc) which is being compressed and encodedindividually. In INTER mode each of the blocks are assigned one orseveral motion vectors. A prediction of the frame is constructed bydisplacing pixel blocks from past and/or future frame(s) according tothe set of motion vectors. Finally, the difference, called the residualsignal, between the frame to be encoded is transformed in a similar wayto the INTRA-mode and together with its motion-compensated prediction isentropy encoded to form a bit stream.

Moreover, in INTER-mode skip blocks may be used. A skip block is codedwithout sending residual error or motion vectors. The encoder will onlyrecord that it is a skip block. The decoder will deduce the motionvector of a skip block from other blocks already decoded. According tothis invention the motion vector of a skip block is preferably deducedfrom a block of a proceeding frame of the digital video data.

Video frames being encoded according to the INTRA-mode, without anyreference to any past or future frame, are called I-frames. Video framesbeing encoded according to the INTER-mode are themselves eithermono-directionally predicted frames, called P-frames and encoded withreference to a past or future frame which is an INTRA- or INTER-frame,or bi-directionally predicted frames, called B-frames and encoded withreference to two or more past or future reference frames. Both P-framesand B frames may include I-blocks which encodes new data not foundanywhere in earlier frames, but usually they are rare.

INTRA-frames comprise either scene change frames, placed at thebeginning of a new group of frames corresponding to a scene change,where no temporal redundancy is available, or refresh frames, placed inother locations where some temporal redundancy is available. I framesare usually inserted at regular or irregular interval to haverefresh-point for new stream encoders or as a recovery point fortransmission errors.

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which currently preferredembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided for thoroughness and completeness, and fully convey the scopeof the invention to the skilled person.

Encoding video images of a comparably large size, e.g. 4K or 8K videoimages, requires more cache memory than what is typically available invideo devices today. This invention addresses this problem with anencoding method which enables video devices to encode comparably largevideo images using cache memories with a limited cache width.

As mentioned the present invention may e.g. be used for encoding 4Kvideo images but the method could of course also be used for otherapplications experiencing a similar problem with a source image having alarger image width (in number of pixels or blocks) than the cache memorycan handle. An example of this is a camera having several image sensorsand fusing images from these image sensors to a composite image that isthen encoded.

For example, in the case of encoding 4K video images it will be possibleto use a video device having a digital video data encoder with a cachememory only capable of encoding 2K video (50% reference cache size). Thesource video is hence 3840×2160 pixels and the cache width is limited to1920 pixels in this example.

In FIG. 1 an example of a sequence of four frames of encoded digitalvideo data corresponding to a sequence of digital source imagesaccording to the present invention is illustrated. The frames of theencoded digital video data are divided into a plurality of blocks 2. Inthe in FIG. 1 shown example each frame of the encoded digital video datais divided into 16 blocks. However, each frame may be divided into anarbitrary number of blocks. Each of the digital source images used forencoding the digital video data have an equal source image widthcorresponding to a first number of blocks. The source images used toencode the digital video data of FIG. 1 have an image widthcorresponding to four blocks. It is however realized that source imageshaving an image width corresponding to any number of blocks (except awidth of one block) may be used. The digital video data encoder used toencode the digital video data has a cache memory with a cache widthcorresponding to a second number of blocks, in the example illustratedin FIG. 1 the cache width corresponds to two blocks. The second numberof blocks is smaller than the first number of blocks. Hence, the cachewidth is smaller than the source image width.

An INTRA-frame 10, being a first frame of the digital video data, isencoded using an INTRA-encoding mode. The INTRA-frame 10 corresponds toa first frame of the sequence of digital source images. The cache memoryis not used for encoding INTRA-frames. The INTRA-frame 10 has an imagewidth corresponding to the source image width. Hence, in the exampleshown in FIG. 1 the image width of the INTRA-frame 10 is 4 blocks wide.

For each INTRA-frame at least one additional frame 20 of the digitalvideo data is encoded using an INTER-encoding mode. The at least oneadditional frame 20 have an image width corresponding to the sourceimage width. The additional frame 20 of the digital video data isencoded as a number of skip blocks. The additional frame 20 of thedigital video data does not correspond to any of the frames of thesequence of digital source images. The amount of additional frames 20depend on the number of parts the subsequent frames of the sequence ofdigital source images are divided into, see below regarding the divisionof the subsequent frames into parts. The amount of additional frames 20are equal to the number of parts into which the subsequent frames of thesequence of digital source images are divided minus one. For example, ifthe subsequent frames are divided into four parts the numbers ofadditional frames are three. In the example illustrated in FIG. 1 theamount of additional frames is one since the subsequent frames of thesequence of digital source images are divided into two parts. Theadditional frame 20 of the digital video data is introduced so that aconstant frame rate is obtained.

To encode a subsequent frame of the sequence of digital source images,being subsequent the first frame of the sequence of digital sourceimages, INTER-encoding mode is used. The subsequent frame is dividedinto parts, in FIG. 1 two parts are used however it is realized that thenumber of parts may vary depending on the cache width and the width ofthe digital source images. The parts of the subsequent frame of thedigital source image have an image width being smaller than or equal tothe cache width such that data pertaining to the parts of the digitalsource image is storable in the cache memory. Thereafter, a frame 30,corresponding to the third frame of the digital video data of FIG. 1 isencoded using INTER-encoding mode. The frame 30 comprises a first regioncomprising data corresponding to a first part of the subsequent digitalsource image and a region padded with skip blocks. In FIG. 1 the blocksof the first region being marked with P and the blocks of the regionpadded with skip blocks being marked with S. After that, a subsequentframe 40, corresponding to the fourth frame of the digital video data ofFIG. 1, comprising a first region padded with skip blocks, the blocks ofthe first region being marked with S in FIG. 1, and another regioncomprising data corresponding to a second part of the subsequent digitalsource image, the blocks of the another region being marked with P inFIG. 1, is encoded using INTER-encoding mode. The image width of theframe 30 and subsequent frame 40 corresponds to the source image width.Hence, in the example of FIG. 1 the subsequent frame of the sequence ofdigital source images is encoded using two separate INTER-frames.

The size of the image parts described in the example may be altered tofit specific requirements of an encoder used for encoding the digitalvideo data and a decoder used for decoding the digital video data. Forexample, if the cache width is smaller than half of the source imagewidth, the source image may be divided in four parts having the sameheight as the source image and an image width being a fourth of thesource image width.

The encoding method according to the present invention may be executedusing existing standard digital video data encoders being able to encodeaccording to any of the ISO/MPEG standards or the ITU-H.26X standards.

By using the encoding method according to the present invention encodeddigital video data is produced, wherein each frame of the encodeddigital video data have an image width corresponding to the image widthof the source image width but a frame rate being a multiple of the framerate of the sequence of digital source images. Such digital video datamay be decoded and played back using a standard decoder and video deviceby setting the frame rate of the video device to the multiple frame rateof the sequence of digital source images and setting the image width tothe source image width.

Hence, according to the example illustrated in FIG. 1, the digital videodata encoder sends a message to the receiver of the digital video datathat the video has full size (in this case having an image width of fourblocks) but two times higher frame rate than the frame rate of thesequence of digital source images.

Alternatively, by using the encoding method of the present invention itis possible to inform the decoder arranged to decode the encoded digitalvideo data about the encoding method which enables the decoder todisplay the decoded encoded video data in an optimal manner. Forexample, by sending information pertaining to a frame rate of theencoded video data and a frame rate of the sequence of digital sourceimages or information pertaining to the cache width and the source imagewidth a ratio between the two frame rates may be found. In the decoderthe ratio between the frame rates may be used to set which frames of thedecoded video data that shall be displayed. If for example 4 times theframe rate of the sequence of digital source images was used to encodethe digital video sequence the decoder may choose not to display newinformation until all four parts of the original sequence has beendecoded. By using this alternative method of displaying the decodedencoded video data the frame rate of the displayed video datacorresponds with the frame rate of the sequence of digital sourceimages.

In the example of FIG. 1 two times the frame rate of the sequence ofdigital source images was used to encode the digital video sequence thedecoder. Hence, only every second frame of the decoded encoded videodata needs to be displayed. More precisely, in the example of FIG. 1 theadditional frame 20 comprising only skip blocks and the subsequent frame40 being the last INTER-frame of the subsequent frame of the sequence ofdigital source images shall be displayed. The skip blocks imply that theimage information from the earlier frame will be displayed.

With reference to FIG. 2 the method according to the present inventionfor encoding digital video data corresponding to a sequence of digitalsource images will be discussed in more detail. Each of the digitalsource images have an equal source image width corresponding to a firstnumber of blocks. The method is performed using a digital video dataencoder comprising a cache memory, the cache memory have a cache widthcorresponding to a second number of blocks, wherein the second number ofblocks is smaller than the first number of blocks. Hence, the cachewidth is smaller than the source image width. The method comprising thefollowing steps:

Encoding, step 201, an INTRA-frame 10 comprising data corresponding to afirst image of the sequence of digital source images.

Providing, step 202, at least one additional frame 20 being anINTER-frame comprising only skip blocks.

Dividing, step 203, a digital source image of the sequence of digitalsource images being subsequent the first source image of the sequence ofdigital source images into a plurality of parts. Each part of thesubsequent digital source image having an image width being smaller thanor equal to the cache width such that data pertaining to each part ofthe digital source image is storable in the cache memory. Preferably,the parts of the subsequent source image are equal in image width.

Encoding, step 204, a frame 30 of the digital video data, the frame 30comprising data corresponding to a first part of the subsequent digitalsource image and skip blocks such that an image width of the first framecorresponds to the source image width. The frame 30 of the digital videodata being encoded using INTER-mode.

Encoding, step 205, a subsequent frame 40 of the digital video data, thesubsequent frame 40 comprising data corresponding to another part of thesubsequent digital source image and skip blocks such that an image widthof the second frame corresponds to the source image width. Thesubsequent frame 40 of the digital video data being encoded usingINTER-mode.

Sending, step 206, side information pertaining to at least one of aframe rate of the encoded video data, a frame rate of the sequence ofdigital source images, the cache width and the source image width to adecoder arranged to decode the encoded digital video data. The sendingof the side information to the decoder may be made over a side channel,or by special messages in the bit stream or by manual configuration orpre configuration.

Decoding can be made without using the side information at all. Hence,the decoder is functioning as normal when decoding an encoded digitalvideo sequence. If decoding is made without using the side informationat all the step 206 of sending the side information to the decoder maybe opt out.

Alternatively, the side information may be used to adjust the displaytime for the frames of the decoded digital video sequence. If so, theside information may be used adjust the display time of the decodeddigital video sequence to match the frame rate of the sequence ofdigital source images. If for example 4 times the frame rate of thesequence of digital source images was used to encode the digital videosequence the decoder may choose not to display new information until allfour parts of the original sequence has been decoded.

In order to inform the decoder of how many frames are used to encode oneframe of the sequence of digital source images information pertaining tothe frame rate of the encoded video data, the frame rate of the sequenceof digital source images or a ratio between these frame rates may beused. Alternatively, the cache width and the source image width or aratio between these two widths may be used.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within thescope of the appended claims.

For example, it is realized that the encoding of the digital video datacorresponding to a sequence of digital source images may be performed byany standard digital video encoder being arranged for encoding accordingto video coding standards of the ISO/MPEG family (MPEG-1, MPEG-2,MPEG-4), the video recommendations of the ITU-H.26X family (H.261, H.263and extensions, H.264 and HEVC, the upcoming h.265 standard). However,the same techniques can easily be applied to other types of video codingstandards, e.g. Microsoft codecs belonging to the WMV-family, On2 codecs(e.g. VP6, VP6-E, VP6-S, VP7 or VP8) or WebM.

The encoder may have a processor having processing capabilities forexecuting a program for implementing the method for encoding the digitalvideo data according to the present invention.

The encoder may have hardware components for performing the steps of themethod for encoding the digital video data according to the presentinvention.

The encoder may have both hardware components for performing some of thesteps of the method for encoding the digital video data according to thepresent invention and a processor having processing capabilities forexecuting a program for implementing some of the steps of the method forencoding the digital video data according to the present invention.

The digital video encoder may be located in a digital video camera beingarranged for capturing the sequence of digital source images.Alternatively, the digital video encoder may be located in a videoencoding device arranged to encode digital source images from externalvideo capturing devices.

Additionally, variations to the disclosed embodiments can be understoodand effected by the skilled person in practicing the claimed invention,from a study of the drawings, the disclosure, and the appended claims.Moreover, in the drawings and specification, there have been disclosedpreferred embodiments and examples of the invention and, althoughspecific terms are employed, they are used in a generic and descriptivesense only and not for the purpose of limitation, the scope of theinvention being set forth in the following claims. In the claims, theword “comprising” does not exclude other elements or steps, and theindefinite article “a” or “an” does not exclude a plurality.

What is claimed is:
 1. A method for encoding digital video data using adigital video data encoder, the method comprising: dividing a digitalsource image of a sequence of digital source images into a plurality ofparts, each part having an image width smaller than or equal to a cachewidth of a cache memory of the digital video encoder, each of thedigital source images having a source image width corresponding to afirst number of blocks; encoding a first frame of the digital videodata, the first frame comprising data corresponding to a first part ofthe digital source image in the sequence of digital source images, thefirst part of the digital source image having an image width beingsmaller than the source image width; and encoding a second frame of thedigital video data, the second frame comprising data corresponding to asecond part of the digital source image, the second part of the digitalsource image having an image width being smaller than the source imagewidth, wherein the first frame and the second frame are encoded asINTER-frames.
 2. The method according to claim 1, wherein the first partof the digital source image and the second part of the digital sourceimage are first and second parts of the same digital source image,wherein the first and second parts are different parts.
 3. The methodaccording to claim 1, wherein the first and second parts of the digitalsource image do not overlap.
 4. The method according to claim 1, whereinthe image widths of the first and second parts are equal.
 5. The methodaccording to claim 1, wherein the cache memory has a cache widthcorresponding to a second number of blocks, wherein the second number ofblocks is smaller than the first number of blocks, wherein the firstpart of the digital source image having an image width corresponding toa third number of blocks being smaller than or equal to the secondnumber of blocks such that data pertaining to the first part of thedigital source image is storable in the cache memory, wherein the secondpart of the digital source image having an image width corresponding toa fourth number of blocks being smaller than or equal to the secondnumber of blocks such that data pertaining to the second part of thedigital source image is storable in the cache memory.
 6. The methodaccording to claim 5, wherein the source image width is a multiple ofthe cache width.
 7. The method according to claim 1, further comprisingsending side information pertaining to the encoded digital video data toa decoder arranged to decode the encoded digital video data, wherein theside information comprises at least one of a frame rate of the encodedvideo data, a frame rate of the sequence of digital source images, thecache width and the source image width.
 8. The method according to claim1, further comprising sending side information of how many frames areused to encode one frame of the sequence of digital source frames.
 9. Anencoder arranged to: divide a digital source image of a sequence ofdigital source images into a plurality of parts, each part having animage width smaller than or equal to the cache width of a cache memoryof the digital video encoder, each of the digital source images having asource image width corresponding to a first number of blocks; encode afirst frame of the digital video data, the first frame comprising datacorresponding to a first part of the digital source image in thesequence of digital source images, the first part of the digital sourceimage having an image width being smaller than the source image width;and encode a second frame of the digital video data, the second framecomprising data corresponding to a second part of the digital sourceimage, the second part of the digital source image having an image widthbeing smaller than the source image width, wherein the first frame andthe second frame are encoded as INTER-frames.
 10. A system comprising: avideo capturing device arranged to capture a sequence of digital sourceimages; and an encoder arranged to: divide a digital source image of thesequence of digital source images into a plurality of parts, each parthaving an image width smaller than or equal to the cache width of acache memory of the encoder, each of the digital source images having asource image width corresponding to a first number of blocks; encode afirst frame of digital video data, the first frame comprising datacorresponding to a first part of the digital source image in thesequence of digital source images, the first part of the digital sourceimage having an image width being smaller than the source image width;and encode a second frame of digital video data, the second framecomprising data corresponding to a second part of the digital sourceimage, the second part of the digital source image having an image widthbeing smaller than the source image width, wherein the first frame andthe second frame are encoded as INTER-frames.
 11. The system accordingto claim 10, wherein the encoder is located in the video capturingdevice.
 12. The system according to claim 10, wherein the encoder islocated in a video encoding device being separate from the videocapturing device.
 13. A system comprising: an encoder arranged to:divide a digital source image of the sequence of digital source imagesinto a plurality of parts, each part having an image width smaller thanor equal to a cache width of a cache memory of the encoder, each of thedigital source images having a source image width corresponding to afirst number of blocks; encode a first frame of digital video data, thefirst frame comprising data corresponding to a first part of the digitalsource image in the sequence of digital source images, the first part ofthe digital source image having an image width being smaller than thesource image width; encode a second frame of digital video data, thesecond frame comprising data corresponding to a second part of thedigital source image, the second part of the digital source image havingan image width being smaller than the source image width, wherein thefirst frame and the second frame are encoded as INTER-frames; and sendthe encoded digital video data to a decoder; and a decoder is arrangedto decode and display the received digital video data.
 14. The systemaccording to claim 13, wherein the encoder is further arranged to sendside information pertaining to the encoded digital video data to adecoder arranged to decode the encoded digital video data, wherein theside information comprises at least one of a frame rate of the encodedvideo data, a frame rate of the sequence of digital source images, thecache width, the source image width, and the number of frames that areused to encode one frame of the sequence of digital source frames; andwherein the decoder is further arranged to display the decoded digitalvideo data based on the received side information.